To use wind energy from wind power plants is an attractive although challenging alternative for power generation. One challenge in this regards relates to the location of wind farms, typically being far from a main power grid. The power from the wind parks has to be delivered to the main power grid in an efficient and reliable manner. High Voltage Direct Current (HVDC) transmission for interconnecting e.g. an off-shore wind park to the on-shore main power grid is considered a suitable alternative.
Operation of parallel voltage source converter (VSC) HVDC links feeding the power from the off-shore wind park to the main land may seem very straight forward in that strategies used for operating parallel generators feeding the main power grid could be used. However, there is a significant difference: mechanical inertia.
In the regular case, wherein generators and motors generate and deliver power to the main AC grid, the generator and motor loads provide a significant mechanical inertia. Frequency variations in the main AC grid are therefore smooth.
In contrast, in a large wind park feeding several VSC converters (rectifiers on the off-shore side) there is no mechanical inertial at all. The power output from the wind turbines is transferred via power converters and the VSC converters, which also lack built-in mechanical inertias. The only built-in inertia in the system lies in the capacitances of cables interconnecting the parallel off-shore VSC converters. The cable capacitances prevent sharp steps in phase angle of the parallel off-shore converters.
The converters in the wind park control the frequency and phase angle of their internal source voltage. This entails the risk of instability between control systems of the parallel-connected off-shore VSC converters.
Parallel-connected off-shore wind parks should be controlled so as to ensure stability of power transmission. Conventional power control using frequency droop can be difficult to implement due to the lack of mechanical inertia. In particular, it can be difficult to obtain stability of such system. Further, the varying wind speed gives an varying power generation and the control of the parallel connected VSC converters has to be able to rapidly enable load sharing during overload as well as during loads below capacity.